W0007
Ammonium Salts of Aromatic Carboxylic Acids. James A.
Kaduk, BP Chemicals, P.O. Box 3011 MC F-9, Naperville IL 60566.
Reaction of ammonia vapor with solid aromatic carboxylic acids
yields low-density salts, characterized by networks of strong N-H...O
hydrogen bonds; some of these salts have open frameworks. Contacting these
salts with water results in formation of ammonium hydrogen salts, which contain
very strong O-H...O hydrogen bonds. These salts are convenient
reagents for the preparation of metal complexes of these acids at low pH, but
turn out to be of interest in their own right.
The crystal structures of diammonium terephthalate
(1,4-benzenedicarboxylate) and diammonium 2,6-naphthalenedicarboxylate were
determined by applying Monte Carlo simulated annealing techniques to synchrotron
powder data. The structures are topologically-equivalent, containing tetrahedral
ammonium synthons and 8-connected carboxylate synthons; layers of
N-H...O hydrogen bonds result in ice-like structures.
Recrystallization from water results in ammonium hydrogen salts, with chains
forms by very strong (O...O = 2.44 Å) O-H...O
hydrogen bonds.
Use of tricarboxylic acids results in more-interesting
chemistry. Reaction of ammonia vapor with trimellitic acid
(1,2,4-benzenetricarboxylic acid) yields a salt which reacts rapidly with
atmospheric water to form
(NH4)2HC9H3O6, which
contains a very strong intramolecular O-H-O hydrogen bond between the 1- and
2-carboxylates. Reaction of ammonia with trimesic acid
(1,3,5-benzenetrocarboxylic acid) yields
(NH4)3C9H3O6(H2O)2,
which has a microporous structure; the water molecules reside in channels
between the hydrogen-bonded cations and anions.
Quantum chemical calculations (density functional plane wave
pseudopotential, using CASTEP) haven proven crucial to determining the hydrogen
positions in these powder structures, and for quantifying the hydrogen bonding.
Besides the normal geometrical parameters, the Mulliken overlap populations seem
to be a useful measure of the strengths of the hydrogen bonds.